Point defects and their effect on dielectric permittivity in strontium titanate ceramics

The origin of dielectric properties of strontium titanate ceramics is investigated using DFT calculations in periodic system. It was determined that the main factors contributing to the increase in dielectric permittivity are: tetragonal distortion of the normally cubic lattice, and charge imbalance induced displacement of titanium center from its central position. Oxygen vacancies were determined to create significantly larger effects than other types of vacancies, like Ti and SrO. The extent of tetragonal distortion was found to be determined by oxygen vacancy distribution, rather than total concentration: relatively symmetrical distribution of oxygen vacancies resulted in smaller tetragonal distortion of the lattice, and, consequently, smaller increase in dielectric permittivity. Charge imbalance naturally destabilizes the cubic lattice, forcing the Ti-atom out of its central position, resulting in tetragonal lattice with increased dielectric permittivity. The process stabilizes the strontium titanate lattice, while increasing the c/a ratio. Therefore, the dielectric permittivity of strontium titanate can be increased by changes to the system that increase tetragonal distortion of the lattice and/or introduce additional negative charge.

Die Kristallstruktur von [(THF)2Li(μ2-Cl)2TiCl2(THF)2] / Crystal Structure of [(THF)2Li(μ2-Cl)2TiCl2(THF)2]

The crystal structure of the title com pound has been determined by X-ray methods. Space group C2/c, Z = 4, lattice dimensions at -80 °C: a = 1343.8(2), b = 1031.6(1), c = 1669.4(1) pm, β = 96.47(1)°. The compound forms an ion pair in which the lithium atom is coordinated in a distorted tetrahedral geometry by two THF molecules and by two chlorine atoms. The halide atoms have a μ2-bridging function, coordinating the lithium and the titanium center. The titanium atom possesses a distorted octahedral environment with two THF ligands in trans-position.